It happens more times to many people than you may be aware – your turn the car ignition to START and instead, you hear the machine gun sound or click because the battery voltage is low, overnight the battery voltage drops.
This post looks at why the car battery voltage may be dropping and suggests some fixes that may help you get your car working reliably again!
Note: The information provided here is only intended as a guide. Always contact a qualified auto mechanic.
Why the Battery Voltage Drops Overnight
If the car battery voltage drops overnight, check:
1. The car ignition may have accidentally been left in the ON position. Now this will drain the car battery flat in a matter of hours!
Did you leave one or more car accessories switched ON? Perhaps the car stereo, the cabin, or the trunk light?
While modern cars use LED lights, over time the draw adds up and can drain the battery.
2. There’s a problem with the car’s electrical system – it could be the alternator not working as it should, a case of loose wire connections to the alternator, battery including weak or loose ground connections.
3. There’s a drain from the car accessories that stay ON even when the ignition is off. If there’s an alarm or onboard sensors that stay active then these can continue drawing the car battery’s power provided they’re wired to the car’s always ON circuit.
Parasitic current draw from car accessories. This drop should not be significant though unless there’s a parasitic appliance that stays ON and continues drawing a significant amount of power in the excess of 30 mA to considerably drop the battery voltage overnight! An example of this is a stuck starter relay.
Tips to Identify the Cause of Parasitic current draw
Note: Do not hesitate to contact a qualified car mechanic
You can use a good-quality clamp meter to monitor the current drawn from the battery. With the ignition switch in the off position, remove the car fuses (fuse box located near the battery) one at a time while monitoring the current draw.
Should the current drop to about 30 mA then bingo! You’ve found the offending circuit and hopefully the faulty accessory.
4. The battery is aging or damaged. It could also be that the battery is damaged or has lost its capacity to hold charge. When this happens the car battery voltage can drop even to 10 volts overnight!
Has your battery been tested at a reputable auto store? how many CCAs is it able to deliver compared to what it is designed to supply?
Use this to inform your decision on whether to replace your battery or not.
How Many Volts Should a Car Battery Lose Overnight
Surface charge aside, a fully charged battery in good condition can hold its voltage or even drop to a fraction of a volt only overnight depending on which car accessories remain switched ON even when the ignition is turned OFF.
A higher voltage drop should not be ignored or else you risk draining the car battery flat and being unable to start.
Contact a qualified mechanic. You can also review the possible causes of battery drain.
*Immediately after driving, the battery voltage might read as much as 12.8 VDC with surface charge and then drops to about 12.6 VDC after 30 or so minutes.
Related Questions
Why a Car Battery Drops to 10 Volts Overnight?
If the car battery voltage drops to 10V overnight, check that the car’s ignition or accessories were not left in the ON position.
It is also possible that the alternator not working as it should, there’re loose wire connections to the alternator and the battery including weak or loose ground connections.
Or the battery is aging or damaged. Contact a qualified auto mechanic.
Ignition is Turned On? What Causes the Battery Voltage to Drop
Note, as the car starts, the starter can draw up to several hundred amps to turn the engine over. When this happens there’ll be a momentary drop in battery voltage that the battery should be able to handle.
That aside, if the car battery voltage drops when the ignition is simply turned on, it is also possible that the alternator not working as it should, and there’re loose wire connections to the alternator and the battery including weak or loose ground connections.
Or the battery is aging or damaged. Contact a qualified auto mechanic.
Closing Thoughts
There’re several possible reasons why the car battery voltage may drop overnight from accidentally leaving your car’s ignition or accessories ON, the alternator not working as it should, a case of loose wire connections to the alternator, battery including weak or loose ground connections, or the battery is aging or damaged.
Always contact a qualified auto mechanic.
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